Electron discharge device



Aug. 1, 1939. c. s. BULL ELECTRON DISCHARGE DEVICE Filed Sept. 10, 1957 ooo vvoaufln Z INVENTOR.

cursor 554 row BULL (6&0;

ATTORNEY.

Patented Aug. 1, 1939 UNITED STATES ATENT OFFiCE to Electric & Musical industries Limited, Hayes, Middlesex, England, a company of Great Britain Application September 10, 1937, Serial No. 163,169 in Great Britain September 18, 1936 1 Claim,

This invention relates to electron discharge devices and has particular, but not exclusive, reference to thermionic valves.

In thermionic valves of the screen grid type and other valves having electrodes disposed in the electron stream and held at a positive potential with respect to the cathode which serve as screening and electron accelerating electrodes, it is desirable to maintain the current taken by such screening electrodes as low as possible since this current serves no useful purpose. It is to be understood that in referring to the current taken by such screening electrodes or screen current is meant the primary electron current flowing to 1| the screening electrode and not the resultant current due to the net effect of the collection of primary electrons and liberation of secondary electrons. Whilst Zero primary current is associated with zero dissipation of power, a zero resultant current may be associated with considerable dissipation of power. In order to reduce the screen current it has been previously suggested, in connection with screen grid valves, to align the turns or elements of the control grid with the turns or elements of the screening grid with respect to the flow of electrons. This arrangement, however, suffers from the disadvantage that in order to reduce the screen current to a minimum the clearance between the control grid and the screening grid must be of the order of magnitude of the pitch of the turns on the grid if the control grid is to operate at zero or negative potentials. The clearance between the grid and screening grid, is, therefore, relatively 85 small with the result that the interelectrode capacity between the grid and the screening grid is relatively high so that the ultimate sensitivity of the valve is impaired.

. It is the chief object of the present invention w to provide an electron discharge device in which improved means are provided for reducing screen current.

According to. the invention the electron discharge device comprises a cathode, a control electrode and an output electrode, a screening electrode disposed between the control electrode and the output electrode, and is characterized in that an additional screening electrode is interposed between the first-mentioned screening electrode and the control electrode, the elements or turns of the two screening electrodes being aligned with respect to one another and to the flow of electrons from the cathode, the distance apart of said screening electrode and additional screening electrode being such that when zero or low negative potential with reference to the cathode is applied to said additional screening electrode and a relatively high potential to the first-mentioned screening electrode, no, or very little, current is collected by the first-mentioned screening elec- 5 trode.

Where the screening electrodes are in the form of helices of wire, it will, in most cases, be found that the condition for minimum screen current is obtained when the difference between the radii m of the two helices is approximately equal to, or less than, the pitch of the turns of wire.

In order that the said invention may be clearly understood and readily carried into effect, the same will now be more fully described with ref- 15 erence to the accompanying drawing in which Figure 1 is a diagrammatic representation showing the arrangement of electrodes in a thermionic valve constructed according to the invention, and

Figures 2 and 3 are circuit diagrams of two amplifier stages each including a thermionic valve constructed according to the invention.

Referring to Figure 1 of the drawing it will be seen that the group of electrodes therein is shown as comprising two cylindrical electrodes, namely the cathode l and the anode 2 located respectively within and surrounding three concentrically arranged grid electrodes each formed of a helical spiral of wire, the innermost grid being the control grid 3 and the outermost being the screening grid 4 and there being an additional grid 5 located between the grids 3 and 4. These elec trodes are mounted within the evacuated envelope 6. The thermionic grids 4 and 5 shown in the drawing are more open than those of the grid 3 and the turns or imperforate portions of these grids are aligned. The pitch of grids 4 and 5 does not necessarily have any relation to that of grid 3. The grid 5 is shown placed closer to the grid t than to grid 3, distance between grid 5 and grid 4 being slightly less than the pitch of the helices of which the two grids are formed. Since there is no particular relation between the pitch of grids i and 5 and that of grid 3, the dis- 45 tance from grid 3 to grid 5 can be chosen to be at least approximately equal to or greater than the distance from the control grid to the screen grid of ordinary known screen grid valves. Furthermore, the distance can be made greater than 50 that from control grid to screen in the type of valve mentioned above, in which the turns of one control grid are aligned with those of the screen grid. Thus a valve according to the invention may have as low a capacity between control grid 55 and screening electrodes as is found between the control grid and screening grid in an ordinary screen grid valve, and considerably lower capacity than is found between control grid and screening grid in the case where these two grids are aligned.

In using the valve, the additional grid 5 will in most cases be maintained at cathode potential, for which purpose the additional grid is preferably connected either internally or externally of the valve envelope to the cathode I. The screening grid 4 is maintained at a positive potential of the order usually applied to screening grids in screen grid valves.

The provision of the additional grid 5 which in use is maintained at cathode or low negative potential in association with the screening grid 4 maintained at a positive potential as above stated serves, it is thought, to focus or deflect the electrons through the apertures or interstices of the positively charged screening grid 4, as indicated by the dotted arrows A, B, C, D and E to the left of Figure 1. These dotted arrows represent electron paths.

It is believed, that as shown, the effect of the additional grid 5 is most marked in respect to electrons following paths such as A, C and D which are such that the electrons travelling along the paths would normally approach the wires of the screening grid, and would probably be attracted thereto. Due to the action of the additional grid 5, which is located only a short distance in front of grid 4, electrons which would approach the screen wires are deflected and do not. fall onto the screening grid. Electrons moving in radial or vertical paths such as B and E are substantially unaffected by the presence of the additional grid 5. The provision of grid 5 as described thus appears to serve to prevent the electrons impinging on grid 4, and hence to reduce or substantially eliminate flow of screen current which usually occurs with existing valves. The method of utilizing a valve according to the invention will be readily understood from the accompanying description with reference to Figures 2 and 3 of the drawing.

Figure 2 shows the use of a valve according to the invention in a high or radio frequency amplifier stage. The valve itself is indicated generally at H), having a cathode H, anode I2, control grid [3, screening grid l4, and additional grid l 5 provided according to the invention. The input to the stage is, as usual, applied between the control grid l3 and the cathode ll over the tuned inductance capacity combination I6, over whicha suitable D. C. bias voltage is applied usually negative as indicated by the sign in the drawing. The cathode H is earthed, as shown, as is also the additional grid I 5, and screening grid I4 is connected over resistance I! to a source of high positive potential of a value usual with screening grids as indicated by the sign below resistance H,

A suitable decoupling condenser I8 is also con nected between the screening grid I4 and earth as shown. The output of the arrangement is derived in a well understood manner, from the anode l2 over the tuned inductance capacity combination 19, the anode being also maintained at a high positive potential as indicated by the sign at the end of the anode lead. The output circuit of the valve is suitably screened from the rest of the valve circuits, the screen being indicated by the dotted line S. With the arrangement of Figure 1, the use of a valve according to the invention is found to be specially advantageous in respect of the reduction of noise to signal ratio in the output circuit. This, it is thought, is due to the fact that the current is not shared between the screening grid and the anode. It is known from experiment that when a current of electrons carrying only small random fluctuations is divided between two electrodes, the sum of the fluctuation of the two partial currents flowing to the two electrodes is greater than the fluctuation of the whole current. It is found that in a screened grid valve of a particular known type, the performance of the valve with respect to the noise to signal ratio is about three times worse than the performance of a similar valve with the screen omitted. This, it is thought, can be attributed to the sharing of the electronic current between the screening grid and anode and it is found that by reducing screen current the noise to signal ratio is improved.

Figure 3 shows the application of a valve ac cording to the invention in the output stage of an audio frequency amplifier.

In this case the valve 20, comprising cathode 2|, anode 22, control grid 23, screening grid 24 and additional grid 25, is arranged like valve ID of Figure 2 with the cathode earthed, the control grid biased negatively with respect to the cathode, the additional grid earthed, and the screening grid and the anode maintained at high positive potentials. The input to the stage is connected between the control grid 22 and the oathode 2|, over the transformer 26. The output circuit of the stage includes the primary winding of output transformer 21 to the secondary of which loudspeaking telephone 28 is connected. The anode 23 and the screening grid 24 are each connected to a common source of positive potential indicated by the sign on the drawing, the anode 23 being connected to said source over the primary winding of transformer 21 and the screening grid 24 being connected to said source over resistance 29 in well understood manner. A combination including resistance 30 and condenser 3! is inserted between the screening grid 24 and earth, and this, together with resistance 29 serves to decouple the screening grid from the signal circuits and to adjust the positive potential applied to the screening grid,

In this case, the advantages of the use of the invention that the inter-electrode capacity of the valve and the noise to signal ratio in the output circuit are reduced are not of prime importance as the stage is a power stage and the signal currents handled are large and of low frequency. However, in this case another important advan tage is obtained. With an output circuit of the character shown in Figure 3, including a tetrode or pentode output valve, which does not have an electrode arrangement according to the invention, the screen current varies during a low frequency cycle in such a manner as to reduce the screen voltage during loud passages if sufficient decoupling is used. This effect gives rise to distortion in the signals in the output circuit before the theoretical maximum powder output is reached and hence if a valve of the tetrode or pentode type is employed with the screening electrode associated with an additional screening electrode according to the invention, since the screen current is considerably reduced and hence less decoupling is necessary, the variation in the small screen current does not greatly reduce the maximum power output available. Thus the drain on the high tension supply from which potential is derived for the anode and screening grid respectively can be reduced, without causing distortion to arise on account of the increase of average screen current during loud passages.

In a valve according to the invention, the screening electrode and the additional screening electrode may be of fiat or cylindrical form the combination may be employed to replace any single screening electrode in any known type of electron discharge device. The invention can also be used with any output arrangement such as an anode, an anti-secondary grid and anode, an anode at a large distance or with anodes and earthed plates. The invention may for example, be applied to frequency changer valves, in which case additional screening electrodes may be associated according to the principles outlined above with the two screening electrodes separating the oscillator grid from signal grid and the latter from the output electrode.

In constructing the two associated screening electrodes, each electrode may be composed of a number of turns of wire helically arranged on suitable supports and welded thereto, the two screens being aligned by relatively adjusting the supports. In other cases, the electrodes may be formed on a suitable perforated insulator coated with metal on both sides, one of the metal coatings constituting the additional grid and being maintained at cathode potential, whilst the other metal coating may serve as the accelerating or normal screening electrode. Where two separate screening electrodes are employed they can be aligned by the methods described in the specification of British Patent 435,559.

The invention is not limited in its application to the usual type of thermionic valve, since it can be also applied to valves of the cathode ray type, in which a beam of electrons is deflected by applying signal potentials to suitable deflection plates, or by applying signal currents to suitably arranged deflecting coils. In this type of valve it is usual to employ screening electrode in the form of apertured diaphragms maintained at suitable positive potentials and one or more of these diaphragms in accordance with the invention may be associated with similar apertured diaphragms maintained at cathode or low negative potential for the purpose of preventing a flow of current in the associated screening electrode.

What I claim as new is- An electron discharge device having a cathode, a control electrode, 'and an output electrode, a screening electrode disposed between the control electrode and the output electrode and an additional screening electrode interposed between the first-mentioned screening electrode and the control electrode, the screening electrodes being helical grids with the turns aligned the difference between the radii of the helices forming the screening electrode and the additional screening electrode being not greater than the pitch of the turns of wire of the helical grids.

CABOT SEATON BULL. 

